1. Technical Field
The present invention relates to a method for estimating the distance (also referred to as range) between a transmitter and a receiver, and to a transmitter and a receiver in which the method is implemented.
2. Background Art
Position determination is an important feature for many applications of wireless networks. It has been proposed either as an additional feature to add value to wireless communication networks (such as enhanced routing or ease in installation for wireless sensor networks) or as a stand-alone feature when location awareness is a goal in itself (such as asset or person tracking applications). More specifically, there is a growing interest to provide position determination features in indoor environments.
Well-established technologies for position determination are mostly addressing outdoor scenarios. Well-known examples are the Global Positioning System (GPS) (see for example Wellenhoff, B., Colli, J., and Lichtenegger, H., “Global positioning system: theory and practice” 4th ed. Springer, 1997) and E-911 for positioning of emergency calls in cellular networks (see for example Sun, G., Chen, J., Guo, W., Ravazi, B., “Signal Processing techniques in network-aided positioning”. Signal Processing Magazine IEEE, Vol. 22 No. 4, pp 12-23, July 2005). In the case of GPS, positioning is based on the estimated time-of-flight of signals transmitted by a satellite constellation and as received by the positioning device. In the case of E-911, positioning is typically based on the observed attenuation of the signals exchanged by the terminal and multiple base stations in the cellular network. While these techniques have adequate performance outdoors and when a sufficiently strong line-of-sight signal is available, they suffer low accuracy in indoor environment. The key reason for this performance degradation is the higher number of reflections and often obstructed line-of-sight path, aka multi-path propagation, which complicates the estimation of the so-called time-of-arrival and hence time-of-flight of the transmitted signal. The reason is that the received signals consists of multiple superimposed attenuated, delayed and phase rotated copies of this signal. Especially due to the narrow-band nature of the transmitted signals, these copies have a wide span in the time-domain and hence tend to overlap. As a result, copies having travelled several meters more than the line-of-sight path can typically not be separated from the latter by the receiver and induce errors in the position estimate of the same order of magnitude.
In order to enable accurate position determination in indoor environments with rich multi-path propagation, techniques based on ultra-wideband signalling have been proposed, for example by Kegen Yu, Oppermann, “UWB Positioning for Wireless Embedded Networks” IEEE Radio and Wireless Conference, 2004, pp 459-462, September 2004, and Gezici, S., Tian, Z., Giannakis, G., Kobayashi, H., Molisch, A., Poor, V. and Sahinoglu, Z., “Localization via Ultra-Wideband Radios” Signal Processing Magazine IEEE Vol. 22 No 4, pp 70-84, July 2005. In this case, the transmitted signals feature a very short span in the time-domain and hence provide improved resolution for separating the line-of-sight propagated copy from the reflected copies of the transmitted signal at the receiver. However, ultra-wideband signalling has serious disadvantages with respect to classical narrowband communication when it comes to the link budget. Specifically, the signal-to-noise ratio at the receiver gets proportionally worse as the bandwidth increases, due to the increased in-band noise bandwidth. Due to this, and also the high carrier frequency at which ultra-wideband communication usually takes place, due to regulatory constraints, the power consumption of such system tends to be significantly higher than that of classical narrowband systems.
Several technologies have been proposed that combine the advantages of both ultra-wideband and narrowband signals for their superior properties with respect to distance estimation accuracy and communication efficiency and simplicity respectively. In U.S. Pat. No. 5,960,047 an alerting signal is sent using a narrowband transmitter, along with the ultra-wideband signal used for distance estimation. It exploits the better link budget provided by narrowband communications to simplify the discovery and improve the range of the ultra-wideband ranging signals. In WO-A-02/088776, again a narrowband transceiver is combined with an ultra-wideband transceiver. In this system, the narrowband signal is used for low-rate data communication, again typically for discovery purposes, whereas the ultra-wideband signal is used for ranging.
The systems known from U.S. Pat. No. 5,960,047 and WO-A-02/088776 however have the following disadvantages. Firstly, they need fully separated radio-transceiver devices for transmitting and receiving the narrowband communications and wide-band ranging signals. Secondly, the signalling happens in distinct frequency bands. Thirdly, the ranging transmissions are in no way backward compatible to the existing means of narrowband communications on top of which they are implemented.